An identification code signal is hidden in a carrier signal (such as an electronic data signal or a physical medium) in a manner that permits the identification signal later to be discerned. The carrier signal can thereby be identified, or some machine responsive action can thereby be taken. The technique...http://www.google.com.tw/patents/US6343138?utm_source=gb-gplus-share專利 US6343138 - Security documents with hidden digital data
